The concentration of a compound within blood of a subject may be measured using invasive techniques, for example obtaining a blood sample and placing the sample on a device that is introduced into a spectrophotometer. Alternatively, the concentration of the compound may be determined non-invasively by placing a part of the body within a receptor that is coupled to a spectrophotometer, as disclosed in for example, U.S. Pat. No. 5,361,758 (Hall et al.) or U.S. Pat. No. 6,741,876 (Seciena et al.).
Clinical studies have revealed that the concentration of certain compounds in a body part of a subject may be used to assess the risk of development of specific medical conditions in that subject. Early detection of these types of risks in a patient permits measures to be taken that may slow or even prevent the onset of these conditions. As an example, it has been determined that elevated concentration of cholesterol is an indication of a risk for coronary disease. Similarly, the determination of blood glucose levels is required by diabetic patients. Therefore, the development of simple, methods for determining the concentration of a compound is of importance.
In U.S. Pat. No. 6,365,363, Parfenov et al. describe a method of indirectly measuring the concentration of cholesterol in the skin of a subject by enzymatically oxidizing the cholesterol in a section of the subject's skin and then quantitating the amount of the hydrogen peroxide by-product stoichiometrically formed in this reaction using a second enzymatic reaction. As a complex series of enzymatic reactions are used in this method to indirectly determine the concentration of cholesterol, the method is both costly and prone to error. In addition, the development of a result using this method is time consuming.
In U.S. Pat. Nos. 6,236,047 and 6,040,578, Malin et al. describe a method for determining the concentration of a blood compound using light in the near infrared range by analysing diffusively reflecting radiation emerging from the irradiated sample. Hall et al. also describe in U.S. Pat. No. 5,361,758 a non-invasive technique for directly measuring the concentration of constituents of blood using light in the near-infrared range.
Invasive methods for the assay of a compound involve lancing a body part to produce a blood sample and placing anywhere from about 2 to about 250 μl, or more, of the sample within a sample holding device are known, for example a slide well (U.S. Pat. No. 4,387,972, Valencia; U.S. Pat. No. 5,800,781, Gavin et al.; U.S. Pat. No. 5,207,984, Kheiri), a sample tab (e.g. WO 00/70350, Samsoondar) or filter paper strip or the like. The sample holder may then be introduced within a receptor of the sample holding device of a spectrophotometer to determine the concentration of the sample (e.g. WO 98/39634, Samsoondar; WO 99/47261, Samsoondar). This process may be awkward, typically require significant sample volumes, and may require the use of additional devices, for example a capillary tube (e.g. U.S. Pat. No. 4,791,938, Valkenburg) to collect the sample from the body part and transfer the sample to the holding device. Furthermore, as the blood sample is being transferred to a sample holding device, the chance for contamination from an exposed blood sample is present.